The Semantic Web is the representation of data on the World Wide Web. It is a collaborative effort led by W3C with participation from a large number of researchers and industrial partners. It is based on the Resource Description Framework (RDF), which integrates a variety of applications using XML for syntax and URIs for naming.

4Introduction Whats in data.out?

Metadata is information about other data--gtresources.

Typically it is a collection of property-value pairs

Property names established by convention

What is my data?

Where is my data?

When was this data created?

How was it created?

Who generated this garbage?

The Semantic Web attempts to define a metadata information model for the Internet to aid in information retrieval and aggregation.

What really is the value of creator? Can I derive it from another class, like person?

Can we provide restrictions and rules for properties?

How can I express the fact that title should only appear once?

Current DC encoding in fact is defined by RDFS.

64Some RDFS ClassesRDFS Resource The RDFS root element. All other tags derive from ResourceRDFS Class The Class class. Literals and Datatypes are example classes.RDFS Literal The class for holding Strings and integers. Literals are dead ends in RDF graphs.RDFS Datatype A type of data, a member of the Literal class. RDFS XMLLiteral A datatype for holding XML data.RDFSProperty This is the base class for all properties (that is, verbs). 65Some RDFS PropertiessubClassOf Indicates the subject is a subclass of the object in a statement.subPropertyOf The subject is a subProperty of the property (masquerading as an object).Comment, Label Simple properties that take string literals as valuesRange Restricts the values of a property to be members of an indicated class or one of its subclasses.isDefinedBy Points to the human readaable definition of a class, usually a URL. 66Sample RDFS Defining ltPropertygtltrdfsClass rdfabout"http//.../some/uri"gt ltrdfsisDefinedBy rdfresource"http//.../some/uri"/gt ltrdfslabelgtPropertylt/rdfslabelgt ltrdfscommentgtThe class of RDF properties.lt/rdfscommentgt ltrdfssubClassOf rdfresource"http//.../Resourcegt lt/rdfsClassgt

This is the definition of ltpropertygt, taken from the RDF schema.

The about attribute labels names this nugget.

ltpropertygt has several properties

ltlabelgt,ltcommentgt are self explanatory.

ltsubClassOfgt means ltpropertygt is a subclass of ltresourcegt

ltisDefinedBygt points to the human-readable documentation.

67RDFS Takeaway

RDFS defines a set of classes and properties that can be used to define new RDF-like languages.

RDFS actually bootstraps itself.

You can express inheritance, restriction

If you want to learn more, see the specification

http//www.w3.org/TR/2003/WD-rdf-schema-20030123/

But dont trust the write up

Concepts are best understood by looking at the RDF XML. English descriptions get convoluted.

If you want to see RDFS in action, see the DC

http//dublincore.org/2003/03/24/dces

68What is DAML-OIL?

RDFS is a pretty sparse

Meant to be extended into more useful languages.

Some missing features, summarized on next table.

DAML-OIL builds on RDF and RDFS to define several more useful properties and classes.

Metadata may be directly embedded in the data, may be separate, may be scavenged by roving agents, .

73Semantic Web and Web Services

Metadata is important for the web.

Most major software companies (IBM, MS, BEA, Oracle, Sun) are more interested in Web Services.

Metadata about Web Services is more important than metadata about documents.

We need a Semantic Grid, not a Semantic Web

Semantic Web research focuses heavily on knowledge representation.

Logical assertions in DAML-OIL, for example

There is an opportunity for more research on management of simple but fragmented information nuggets.

RDF nuggets scattered across the web, linked with URIs.

Sophisticated queries can be made over distributed fragments.

74Semantic Web Services as a Trend

The real problem in the Defense Department and in the technology world is not encoding ontological knowledge the real problem is semantic integration across the thousands of databases and software packages.

We have a collection of codes, visualization tools, computing resources, and data sets that we want to combine in an ontology.

Instances of the ontology can then be made that describe specific resources.

After we have built instances, we can pose queries on the data to retrieve values.

Values may be structured, so we can do stepped queries.

We thus need to start by grouping together related resources.

http//grids.ucs.indiana.edu/maktas/servo/index.html

77Group 1 Simulation Codes

Disloc calculates surface stress displacements causes by a fault placed in an elastic half-space. Surface data can be either on a grid or on defined scattered points. Can also create InSAR-style surface displacements.

Simplex inverts Disloc to estimate fault parameters from observed surface displacements. Surface displacements can be either on a grid or at defined points.

GeoFEST does a realistic model of stresses created by a fault. Uses finite element method, realistic material properties.

We now wish to create classes and properties that we can couple into an ontology.

First, lets define a base object, GEMObject, that we will extend as necessary.

This object doesnt do anything but it will have some uses when we define property ranges and domains.

lt?xml version"1.0"?gt

ltrdfRDF xmlnsrdf"http//www.w3c.org/1999/02/22-rdf-syntax-ns"

xmlnsrdfshttp//www.w3c.org/2000/01/rdf-schemagt

ltrdfDescription rdfID"GEMObject"gt

ltrdftype"http//www.w3c.org/2000/01/rdf-schemaClass"/gt

ltrdfslabelgtGEMObjectlt/rdfslabelgt

ltrdfscommentgtThis is a generic object from which everything in our

ontology will be derived.

lt/rdfscommentgt

lt/rdfDescriptiongt

lt/rdfRDFgt

84ltDescriptiongt

The ltRDFgt tag is followed by the ltDescriptiongt tag. This serves two important purposes

A ltDescriptiongt surrounds the property and values for a class.

It also identifies the thing that the description applies to.

The thing is actually a resource and is identified by either a local or absolute URI.

85Defining Some Useful Classes

Based on our introductory comments, we need the following classes

GEMCodes, with application and visualization extensions

GEMData, such as Faults, GPS, and so on.

GEMDataFormat either grid or point data

ComputeResources host computers.

86Example Defining a GEMCode

GEMCodes should extend our GEMObject generic superclass.

It should itself be extended by other, more specific resource types.

ltrdfDescription rdfID"GEMCode"gt

ltrdftype"http//www.w3c.org/2000/01/rdf-schemaClass"/gt

ltrdfssubClassOf rdfresource"GEMObject"/gt

ltrdfslabelgtGEMCodelt/rdfslabelgt

ltrdfscommentgtThis is a general code class that we will extendlt/rdfscommentgt

lt/rdfDescriptiongt

87Defining Properties

Classes by themselves dont tell us much, but when we associate them with properties, things start to fall into place. Before describing how a property may be encoded, lets try and enumerate the ones that we will need.

ownsGEMResource who owns a particular resource.

installedOn, hasCode where a GEMCode is installed, or, conversely, what codes a particular computing resource has.

hasData where some piece of data is (on some compute resource).

hasDataFormat associate a data format with a piece of data.

takesInputData, createsOutputData what kind of data a code takes as input and generates as output.

dependsUpon a code depends upon another operation before it can be completed.

88A Property for Resource Ownership

Now lets look at how to encode this first property. It looks like this

ltrdfDescription rdfID"ownsGEMResource"gt

ltrdftype

rdfresource"http//www.w3c.org/1999/02/22-rdf-syntax-nsProperty"/gt

ltrdfsdomain rdfresource"http//www.w3c.org/2001/vcard-rdf/3.0"/gt'

ltrdfsrange rdfresource"GEMObject"/gt

lt/rdfDescriptiongt

89More Information

The W3C Semantic Web Activity

http//www.w3.org/2001/sw/

The Dublin Core Metadata Initiative

http//dublincore.org/

For some survey reports

http//www.servogrid.org/slide/GEM/SW

See these reports for longer discussions, examples, and references.

For programming examples, see http//www.servogrid.org/slide/GEM/SW/Examples

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